Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. This totality of multiple combined wavelengths comprises a single transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or "channels", typically by a wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on "loops" within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150 which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop 110 which connects with loop 150 at connector point 140. Thus, if local loop 110 is Sacramento, wavelengths at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point 140, and the new signal will travel forward to New York. Within loop 110, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop 110 to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal into its component channels is typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems 200 and 210 with dense wavelength division multiplexers 220 and 230. An optical signal from Loop 110 (.lambda..sub.1 -.lambda..sub.n) enters its add/drop system 200 at node A (240). The signal is separated into its component channels by the dense wavelength division multiplexer 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, .lambda..sub.1 would travel along path 250-1, .lambda..sub.2 would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (.lambda..sub.1 '-.lambda..sub.n ') enters its add/drop system 210 via node C (270). The signal is separated into its component channels by the wavelength division multiplexer 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, .lambda..sub.1 ' would travel along path 280-1, .lambda..sub.2 ' would travel along path 280-2, etc.
In the performance of an add/drop function, for example, .lambda..sub.1 is transferred from path 250-1 to path 280-1. It is combined with the others of Loop 150's channels into a single new optical signal by the dense wavelength division multiplexer 230. The new signal is then returned to Loop 150 via node D (290). At the same time, .lambda..sub.1 ' is transferred from path 280-1 to path 250-1. It is combined with the others of Loop 110's channels into a single optical signal by the dense wavelength division multiplexer 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110's frame of reference, channel .lambda..sub.1 of its own signal is dropped to Loop 150 while channel .lambda..sub.1 ' of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's frame of reference. This is the add/drop function.
Conventional methods used by wavelength division multiplexers in separating an optical signal into its component channels include the use of filters and fiber gratings as separators. A "separator," as the term is used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is the precision required of a device for transmitting a signal into an optic fiber. A signal entering a wavelength division multiplexer must conform to a set of very narrow pass bands. FIG. 3 shows a sample spectrum curve 310 comprised of numerous channels as it enters a dense wavelength division multiplexer. The pass bands 320 of the channels are very narrow. Ideally, the curve would be a square wave. A narrow pass band is problematic because, due to the physical limitations and temperature sensitivity of signal source laser devices, they never emit light exactly at the center wavelength of an optical filter. The difference between the actual wavelength and the wavelength at the center of the pass band is called the "offset." The amount of offset or change in offset ("drift") ideally should not be larger than the width of the pass bands. Otherwise, crosstalk between channels will be too large. Crosstalk occurs when one channel or part of a channel appears as noise on another channel adjacent to it. Since the signals resulting from the conventional wavelength division multiplexer configurations have narrow pass bands, the signal source devices ("transmitter"), such as lasers or the like, must be of a high precision so that offset or drift is limited to the width of the pass bands. This high precision is difficult to accomplish. Signal transmitting devices of high precision are available but are very expensive. Also, the signal transmitting devices must be aligned individually for each separator, which is time intensive. Additionally, the spectrum of pass bands of conventional separators, such as conventional band pass filter, have rounded shapes. Concatenation of several such filters in series inevitably reduces the overall pass band widths and increases the insertion losses of the filter ensemble because of cancellation of energy at the edges of the overlapping individual pass bands.
A more advanced related art separator utilizing a polarization beam splitter and a non-linear interferometer is disclosed in a co-pending U.S. Patent Application entitled "Non-Linear Interferometer for Fiber Optic Wavelength Division multiplexer Utilizing a Phase Differential Method of Wavelength Separation," Ser. No. 09/247,253, filed on Feb. 10, 1999. Applicant hereby incorporates this patent application by reference.
FIGS. 4 through 6 illustrates a preferred embodiment of the separator disclosed in U.S. patent application Ser. No. 09/247,253. This separator 1000 separates the signal into two sets of channels. FIG. 4 illustrates a top view of a preferred embodiment of a separator 1000. The separator 1000 comprises an optical fiber 1010 for inputting an optical signal and optical fibers 1020 and 1030 for outputting optical signals. As the signal leaves the optic fiber 1010, it diverges. A lens 1050 collimates the signal and directs it towards a beam splitter 1070 which decomposes the signal based upon its polarity. This decomposition takes place at a plane 1075 of the beam splitter 1070. The component (p-component) of the input signal polarized within the plane defined by the input signal's direction of travel and a line perpendicular to junction plane 1075 passes through beam splitter 1070 towards an interferometer 800B. The component (s-component) of the input signal polarized parallel to junction plane 1075 is reflected towards an interferometer 800A. The interferometers 800A and 800B introduce phase differences between the even and odd channels of the signals.
FIG. 5 illustrates the path of the light of the odd channels as it travels through the separator 1000 with the interferometer 800A and 800B of the related art invention. The light of the odd channels travels to the polarization beam splitter 1070 from the input fiber 1010. The light of each channel has an s polarity component (E.sub.s) 1110 and a p polarity component (E.sub.p) 1220. The E.sub.s and E.sub.p signals may each be decomposed into E.sub.o and E.sub.e components parallel to the principal ray directions of the birefringent elements in interferometer 800A and 800B, respectively. These components are well known in the art and will not further be described here. The vector E.sub.p 1220 is decomposed into components E.sub.po 1230 and E.sub.pe 1240 whereas the vector E.sub.s 1210 is decomposed into components E.sub.so 1250 and E.sub.se 1260. This decomposition is illustrated in FIG. 5 for each of the signal polarization component vectors E.sub.s and E.sub.p both before its entry into and after its exit from the interferometer 800A and 800B, respectively. The signal E.sub.p 1220 travels to the interferometer 800B while E.sub.s 1210 travels to interferometer 800A. Both sets of signals are reflected by their interferometers 800A and 800B without a phase shift difference between E.sub.so 1250 and E.sub.se 1260 (or between E.sub.po 1230 and E.sub.pe 1240). Thus, both the signal E.sub.p 1220 and the signal E.sub.s 1210 travel back to the polarization beam splitter 1070 without a change in orientation. These signals then travel back through the polarization beam splitter 1070 to output fiber 1020.
FIG. 6 illustrates the path of the even channels as they travel through the separator 1000 with the interferometer 800A and 800B of the present invention. As with the odd channels, the light of the even channels travels to the polarization beam splitter 1070 from the input fiber 1010. The light of each channel has an s polarity component (E.sub.s) 1210 and a p polarity component (E.sub.p) 1320. As with the odd channels, the E.sub.s and E.sub.p signals may each be decomposed into E.sub.o and E.sub.e components parallel to the principal ray directions of the birefringent elements in interferometer 800A and 800B, respectively. The vector E.sub.p 1320 is decomposed into components E.sub.po 1330 and E.sub.pe 1340 whereas the vector E.sub.s 1310 is decomposed into components E.sub.so 1350 and E.sub.se 1360. This decomposition is illustrated in FIG. 6 for the polarization plane of the light of each of the signal vectors E.sub.s and E.sub.p both before its entry into and after its exit from the interferometer 800A and 800B, respectively. The signal E.sub.p 1320 travels to the interferometer 800B while the signal E.sub.s 1310 travels to interferometer 800A. For the even channels, interferometers 800A and 800B introduce a .pi. phase difference between E.sub.po 1330 and E.sub.pe 1340 and also between E.sub.so 1350 and E.sub.se 1360 respectively. This phase difference causes an effective .pi./2 rotation of each of the signals 1310 and 1320, thereby converting them from E.sub.s into E.sub.p and from E.sub.p into E.sub.s, respectively. When both of these signals travel through beam splitter 1070 again, this rotation causes them to travel to output fiber 1030. Thus, in this manner, output fiber 1020 contains the odd channels while output fiber 1030 contains the even channels.
This separator has advantages over conventional separators in terms of increased widths and flatness of the pass bands and isolation bands and greater ease of alignment. Although the separator 1000 is useful for its stated purpose, it may be limited, in some cases, by the properties of the polarization beam splitter used 1070. A perfect polarization beam splitter will separate an incident unpolarized light beam into two plane polarized component light beams with mutually perpendicular polarization orientations such that each component beam comprises 100% of the light of one polarization orientation and none of the light of the other orientation. However, in real beam splitters, which can never be perfect, there is always a small amount of leakage of light rays of one polarization orientation into the pathway nominally comprised only of light with the other polarization orientation. Because of this leakage, there will be imperfect isolation of one set of signals from another in the separator 1000.
Accordingly, there exists a need for a separation mechanism which would allow a wavelength division multiplexer to have a greater tolerance for wavelength offset and drift and a greater ease of alignment than is realized by conventional separators and also a greater degree of isolation between the two sets of separated channels. The present invention addresses such a need.